The present invention relates to a method for recovering heat from spent cooking liquor, such as black liquor, in a digester plant of a chemical pulp mill and for heating white liquor or a mixture of white liquor and another liquid for use in a digester system of the chemical pulp mill.
In a conventional fiberline system with a chip bin where steaming of wood chips or other cellulose material (collectively referred to as “wood chips”) occurs, liquid is added to form a slurry, and the slurry is pressurized, in what is typically referred to as the feed system. From the feed system, the pressurized slurry flows to a treatment vessel(s), such as an impregnation vessel, a pre-hydrolysis process and a digester, in a cooking system for the wood chips.
At least one black liquor stream (typically at a temperature of 110-150° C.) is withdrawn from the conventional cooking system. The extracted black liquor stream(s) is used as a source of heat to “pre-heat” white liquor, other black liquor streams or other liquid streams in the feed and cooking systems. The extracted black liquor stream(s) is then sent to the pre-evaporation system, e.g., two or more flash tanks where steam is produced from the hot black liquor as the liquor is cooled, typically to temperatures of approximately 95-110° C. At this point, the black liquor is sent to the evaporator system in the recovery area. The flash steam is typically used directly to presteam chips prior to cooking.
Steam or vapor is needed for heating the fibrous material to the cooking temperature in the vapor phase of the digester and heating different liquids, such as cooking liquor, white liquor, to a temperature required by the process.
Cooking liquor, such as white liquor, is typically heated by passing it into indirect heat exchange relationship with hot black liquor extracted from a digester or with steam. U.S. Pat. No. 5,635,026 and EP 477,059, for instance, disclose systems in which white liquor added to a digester is heated in an indirect heat exchanger by steam or with hot spent cooking liquor extracted from the digester.
A system is revealed in U.S. Pat. No. 6,176,971 for the generation of clean steam to be used in the digester system. Substantially clean useable steam is produced from a hot spent treatment liquor (e.g. black liquor) by passing the spent liquor to a reboiler, and then pressurizing (e.g. with an eductor, fan, or compressor) the clean steam discharged from the reboiler. The quantity of clean steam produced is increased by placing under negative pressure the clean steam side of the steam converter in the steam converter with a steam-driven ejector. The reduced pressure of the clean steam side ensures that more heat can be withdrawn from the black liquor, which in itself gives a greater quantity of steam, while the supply of steam to the ejector also contributes to the delivery of greater quantities of steam.
A further process for the generation of steam is revealed in U.S. Pat. No. 6,306,252, where the black liquor from the digester is led through a heat exchanger in which e.g. clean process water is heated, after which the pressure of the heated process water is reduced in a flash tank, such that clean steam is generated. The steam can be used for presteaming wood chips or for indirect preheating of cooking liquors or filtrates that are used in and around a digester, for example, white liquor. According to one embodiment of U.S. Pat. No. 6,306,252 black liquor from a continuous digester passes through one or two boilers in which clean liquid, such as condensate, is boiled for generating steam. The black liquor exits the boiler and is further cooled in a cooler in which the coolant may be white liquor, black liquor or green liquor.
US Patent Application Publication 2007/0131,363 discloses a method which comprises generating black liquor in a digester system, sending the black liquor to an evaporator system without using any pre-evaporator system, flashing the black liquor in the evaporator system to yield steam. At least some of the flashed steam is used for chip steaming in a chip bin and/or for supplying in-direct heat exchangers in the digester system for pre-heating white liquor and/or filtrates for use in the digester system.
Conventional solutions provide different systems for pre-heating white liquor and filtrates for use in the digester system and for improving the energy economy of the pulp mill. In the known systems fresh steam from the mill's turbine plant is often used for this purpose. Due to the cost of energy, any further improvement to the energy efficiency of the chemical pulp mill is needed. The heat content of hot black liquor extracted from a digester could also be used in more efficient ways and the properties of the black liquor could be improved so that the liquor is in a better form for further treatment, such as for evaporation in the mill's recovery plant.